Electrolysis with precious metalcoated titanium anode



United States Patent Antilles No Drawing. Filed Mar. 28, 1958, Ser. No. 724,499

Claims priority, application Netherlands, Apr. 9, 1957,

i2 Qlaims. lei. 2%4-98) As is known anodes for carrying out electrolyses and other electrochemical processes usually consist of a precious metal e.g. platinum. Said anodes are quite satisfactory but their high cost price constitutes a bar to their being used in the art on a large scale.

In order to obviate this draw-back it is possible to use an anode consisting of a core of a less precious or base metal coated with a layer of precious metal, mostly platinum. In some cases said layer need only be extremely thin and it may then be applied, e.g., galvanically. Good results, however, are only obtained if the core metal in the places where the layer of precious metal is porous is provided with an inert barrier layer prior to or during the electrolysis.

Thus an anode is known which consists of a core of tantalum coated with platinum while also a core of Zirconium or of a zirconium alloy coated with platinum has been proposed. In both cases the platinum coating may be applied galvanically. Another example is an electrode of bismuth coated with a layer of platinum.

The fact that such electrodes have a high anodic contact resistance must be based on the fact that during the pas sage of the current the base metal of the core is coated with an inert barrier layer in those places where said metal can come into contact with the reaction medium owing to the porosity of the coating, which barrier layer is not easily attacked by the reaction medium and cannot dissolve therein either. Such a barrier layer, therefore, consists of a non-porous film which protects the subjacent base metal against the reaction medium (electrolyte) and which generally speaking does not allow the current to ass.

p I have now found that titanium is particularly suited to be used as a core metal for an anode becaue it has appeared that titanium is not only capable 01f forming a barrier layer in aqueous solutions of substantially all electrolytes, whereas other metals such as bismuth, tantalum and zirconium will only do this it oxygen is evolved directly at the anode, but because when using titanium in addition a very stable barrier layer is formed which is very resistant and which upon prolonged use continues performing its function in contradistinction to the above mentioned known anodes, the barrier layer of which is less resistant and will soon decompose in all kinds of electrolytes eg. in those containing halides.

The invention, therefore, likewise relates to an anode having a core of a base metal provided with a coating of a precious metal or another electricity conducting resistant material which core is or may be electrically provided with a barrier layer in the places Where the coating is porous.

According to the invention the core consists of solid, massive substantially non-porous titanium.

The barrier layer may be formed in situ e.g. in the electrolytic bath, in which the coated electrode is to serve as anode, or the barrier layer may be applied to the core metal by a pretreatment after said core metal has been coated.

The barrier layer may be formed by the electrolysis of a solution of an acid, a base or a salt, inclusive of electrolytes containing halide ions, with the exception, however, of fluorides.

The barrier layer is preferably applied by a pretreatment and electrolytically. In providing the barrier layer it is recommendable to form the barrier layer at a higher voltage than the voltage to which the anode will be subjected in ordinary use. This will ensure that the barrier layer will remain intact in use.

The coating of the electrode need not necessarily consist of a precious metal. It is only necessary for the coating to consist of a material which properly adheres to titanium, conducts the electric current and is not attacked by the electrolyte. As such, for example, a magnetite layer is suitable. Also in this case the porosity of the coating is not objectionable because in the porous places the titanium is provided with a barrier layer.

It should be noted that magnetite electrodes are known. The known magnetite electrodes consist of an iron core to which a magnetite layer has been applied, e.g., by oxidation. In connection with its porosity said layer must be rather thick (two millimeters or more) and it then offers a great resistance to the passage of the current because magnetite is not a first class conductor. If the anode according to the invention is coated with magnetite said coating may be very thin, e.g., about half a micron. Also the thickness of the coating of precious metals used according to the invention may generally be in said order of magnitude. Preferably the thickness of the coatings amounts to about one micron. For elucidating the invention a few examples are subjoined.

Example 1 If titanium is introduced into an aqueous solution of a chloride, e.g., in hydrochloric acid and the titanium is connected as anode the passage of current will be reduced to substantially zero within a few seconds because the titanium is coated with a protective layer which renders any further passage of current impossible and fully protects the subjacent material. If a plate of titanium is coated with a layer of rhodium having a thickness of 1 micron, and if said plate of titanium thus coated is again connected as anode in a hydrochloric acid solution the passage of current will continue undisturbed while the pores in the rhodium are not harmful to the subjacent titanium because this is protected by the film of oxide which will be locally formed. Said electrode is preeminently suited for the electrolysis of alkali chloride solutions because there is no question here of wear and tear, the current density may be a few times larger than in the case of the known electrodes, while there is no pollution at all of the electrolyte, so that there is a considerable economization in cost of maintenance.

In addition a much greater number of said electrodes may be placed in a certain space in the bath because in comparison with the thick graphite or magnetite electrodes the diameter of the electrode according to the invention is appreciably smaller. Because in the alkali chloride electrolysis a continuous movement of the bath is of great importance said electrodes may be perforated, llf desired, so that a hiah rate of flow can be obtained.

Example 2 The electrode of titanium described in Example 1 and coated with a layer of rhodium of 1 micron can also be used for the electrolysis of salt melts. To this end said electrode is arranged in an aqueous solution of 20% bydrochloric acid and is connected as anode, while a plate of carbon serves as cathode. The voltage between said two electrodes is gradually raised to volts (direct current voltage) and is maintained at said level until in the porous places the plate of titanium is coated with a bar- O rier layer through which substantially no current will pass any longer.

This electrode is placed in a melt of zinc chloride which is heated at 330 C. and subsequently the plate is connected as anode and a plate of carbon is used as cathode. When the passage of current is sufficient the mixture will remain at the melting temperature without external heating, while in the case of an insufiicient passage of current heat has to be supplied from the outside. Chlorine will evolve at the anode and zinc Will deposit on the cathode. It is possible in this manner to obtain very pure zinc.

It stands to reason that this anode can also be used for other salt melts. The use of titanium as a core metal is extraordinarily attractive in this case because the melting temperature of titanium is in the neighbourhood of 1800 C. and that of rhodium upwards of 1900 C. By means of this electrode therefore it is possible to work at very high temperatures without the anode being damaged. The carbon graphite electrodes conventionally used for this purpose are much more sensitive to temperature and in consequence will be subjected to an appreciable wear.

Example 3 A plate of titanium is pickled for cleaning purposes by submerging it for 30 seconds in an aqueous solution of 5% ammoniumbifiuoride. Subsequently it is rinsed and coated with an extremely thin layer of iron. This may be effected for example by connecting it as cathode in a bath containing 100 cc. of water, 25 g. of ferrous sulphate and 22 g. of sodium sulphate, while a plate of pure iron is used as anode. The titanium thus coated is rinsed again and dried. Subsequently the plate is heated at 1350 C. for 60 minutes, care being taken that only a limited amount of oxygen is present, the iron being converted then in ferrosoferric oxide or magnetite (Fe304). Care should be taken that no ferrioxide will form because ferrioxide is not a conductor Whereas magnetite is, especially if applied in a thin coating to a conductor. After said electrode has been provided with a barrier layer in the manner described in Examples 1 or 2, it may be used for various electrolyses, more particularly in the alkali chloride electrolysis, because magnetite is highly resistant against chlorine, while in the porous places the titanium is protected from being attacked by chlorine.

I claim:

1. A method of using an electrode consisting essentially of a core of solid, massive substantially nonporous titanium, a porous layer of at least one electricity-conducting electrolyte resistant metal taken from the group consisting of platinum and rhodium, on said core, and an anodically formed barrier layer of titanium oxide on said core at the places where said layer is porous, said method comprising the steps of inserting the electrode into an electrolyte containing halide ions from the group consisting of chloride, bromide and iodide, and passing a direct current through the electrode and electrolyte with the electrode as an anode.

2. A method of using an electrode consisting essentially of a core of solid, massive substantially non-porous titanium, a porous layer of at least one electricity-conducting electrolyte resistant metal taken from the group consisting of platinum and rhodium, on said core, and an anodically formed barrier layer of titanium oxide on said core at the places where said layer is porous, said method comprising the steps of inserting the electrode into an electrolyte containing chloride ions, and passing a direct current through the electrode and electrolyte with the electrode as an anode.

3. A method of using an electrode consisting essentially of a core of solid, massive substantially non-porous titanium, a porous layer of at least one electricity-conducting electrolyte resistant metal taken from the group consisting of platinum and rhodium, on said core, and an anodically formed barrier layer of titanium oxide on said core at the places where said layer is porous, said method comprising the steps of inserting the electrode into an electrolyte containing an alkali chloride, and passing a direct current through the electrode and electrolyte with the electrode as an anode.

4. A method of carrying out electrolysis, comprising inserting an electrode consisting essentially of a core of solid, massive, substantially non-porous titanium, a porous layer of at least one electricity-conducting electrolyte resistant metal taken from the group consisting of platinum and rhodium, on said core, and an anodically formed barrier layer of titanium oxide on said core at the places where said layer is porous, into an electrolyte containing halide ions from the group consisting of chloride, bromide and iodide, and passing a direct current through the electrode and electrolyte with said elect-rode as an anode.

5. A method of carrying out electrolysis, comprising inserting an electrode consisting essentially of a core of solid, massive, substantially non-porous titanium, a porous layer of at least one electricity-conducting electrolyte resistant metal taken from the group consisting of platinum and rhodium, on said core, and an anodically formed barrier layer of titanium oxide on said core at the places where said layer is porous, into an electrolyte containing chloride ions, and passing a direct current through the electrode and electrolyte with said electrode as an anode.

6. A method of carrying out electrolysis, comprising inserting an electrode consisting essentially of a core of solid, massive, substantially non-porous titanium, a porous layer of at least one electricity-conducting electrolyte resistant metal taken from the group consisting of platinum and rhodium, on said core, and an anodically formed barrier layer of titanium oxide on said core at the places where said layer is porous, into an electrolyte containing an alkali chloride, and passing a direct current through the electrode and electrolyte with said electrode as an anode.

7. A method of carrying out electrolysis, comprising coating a core of solid, massive, substantially non-porous titanium with a porous layer of at least one metal taken from the group consisting of platinum and rhodium, anodically oxidizing the titanium which is exposed through the pores of said layer of metal, inserting said coated and anodically oxidized titanium core into an electrolyte containing halide ions from the group consisting of chloride, bromide and iodide, and passing a current through the coated and anodically oxidized core and into said electrolyte, said core serving as an anode.

8. A method of carrying out electrolysis, comprising coating a core of solid, massive, substantially non-porous titanium with a porous layer of at least one metal taken from the group consisting of platinum and rhodium, anodically oxidizing the titanium which is exposed through the pores of said layer of metal, inserting said coated and anodically oxidized titanium core into an electrolyte containing chloride ions, and passing a current through the coated and anodically oxidized core and into said electrolyte, said core serving as an anode.

9. A method of carrying out electrolysis, comprising coating a core of solid, massive, substantially non-porous titanium with a porous layer of at least one metal taken from the group consisting of platinum and rhodium, anodically oxidizing the titanium which is exposed through the pores of said layer of metal, inserting said coated and anodically oxidized titanium core into an electrolyte containing alkali chloride, and passing a current through the coated and anodically oxidized core and into said electrolyte, said core serving as an anode.

10. A method of producing chlorine comprising inserting an electrode consisting essentially of a core of solid, massive, substantially non-porous titanium, a porous layer of at least one metal taken from the group consisting of platinum and rhodium, on said core, and an anodically formed barrier layer of titanium oxide on said core at the places where said layer is porous, as an anode into an 5 electrolyte containing chloride ions, passing a direct current through said electrode and then into said electrolyte, inserting a cathode into said electrolyte, and passing said current out of said electrolyte through said cathode, whereby free chlorine is produced at said anode.

11. An electrolytic process for the preparation of a chemical product, said process comprising the steps of providing an aqueous solution of an alkali metal chloride in an electrolytic cell including an electrode positioned within said solution, said electrode comprising an operative surface layer of platinum on an electrically conducting titanium support, passing an electrolyzing current through the electrode and electrolyte with the electrode as an anode, and recovering said chemical product from said electrolyte.

12. An electrolytic process for the preparation of a chemical product, said process comprising the steps of providing an electrolyte containing halide ions selected from the group consisting of chloride, bromide and iodide in an electrolytic cell including an electrode positioned within said electrolyte, said electrode comprising an operative surface layer of an electrolyte-resistant precious 6 metal on an electrically conducting support consisting essentially of titanium, passing an electrolyzing current through the electrode and electrolyte with the electrode as anode and recovering said chemical product from said electrolyte.

References Cited by the Examiner UNITED STATES PATENTS 1,077,920 ll/ 1913 Stevens 204290 1,477,099 l2/l923 Baum 204290 2,631,115 3/1953 FOX 20429O 2,636,856 4/ 1953 Suggs et al. 204-290 2,719,797 10/ 1955 Rosenblatt et al. 204-290 X 2,865,832 12/ 1958 Pitzer 204-222 FOREIGN PATENTS 236,579 6/ 1945 Switzerland.

JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, Examiner. 

12. AN ELECTROLYTIC PROCESS FOR THE PREPARATION OF A CHEMICAL PRODUCT, SAID PROCESS COMPRISING THE STEPS OF PROVIDING AN ELECTROLYTE CONTAINING HALIDE IONS SELECTED FROM THE GROUP CONSISTING OF CHLORIDE, BROMIDE AND IODIDE IN AN ELECTROLYTIC CELL INCLUDING AN ELECTRODE POSITIONED WITHIN SAID ELECTROLYTE, SAID ELECTRODE COMPRISING AN OPERATIVE SURFACE LAYER OF AN ELECTROLYTE-RESISTANT PRECIOUS METAL ON AN ELECTRICALLY CONDUCTING SUPPORT CONSISTING ESSENTIALLY OF TITANIUM, PASSING AN ELECTROLYZING CURRENT THROUGH THE ELECTRODE AND ELECTROLYTE WITH THE ELECTRODE AS ANODE AND RECOVERING SAID CHEMICAL PRODUCT FROM SAID ELECTROLYTE. 